1. Field of the Invention
The present invention relates to a method of manufacturing a sintered body of indium tin oxide (ITO), particularly a method of manufacturing an ITO sintered body of improved sinterability characteristics.
2. Description of the Prior Art
An ITO sintered body is generally employed in manufacturing transparent conductive electrodes of a flat display device, e.g., a liquid crystal display device. The ITO transparent conductive electrode is generally formed by sputtering which employs an ITO sintered body target. The ITO sintered body is manufactured by mixing indium oxide powder with tin oxide powder in a predetermined ratio, pressing and molding the mixed powder to a predetermined shape, and then sintering the mixture in a sintering furnace. The mixture of indium oxide powder and tin oxide powder, or indium tin oxide powder, i.e., the ITO powder, which are used as raw materials for an ITO sintered body, are processed by thermal decomposition of metal hydroxides, organic metal salts or inorganic metal salts, sols or gels of such metals, or calcining ITO precursors obtained by means of coprecipitation, hydrolysis or precipitation under a temperature range of about 400.degree. C. to 1,000.degree. C.
An ITO sintered body is conventionally known to have superior characteristics when the weight ratio of its composition, i.e., indium oxide powder and tin oxide powder, is 90 wt %:10 wt % and when its density approaches its theoretical density (7.15 g/cm.sup.3 ; IN.sub.2 O.sub.3 =10 wt %; SnO.sub.2 =90 wt %).
FIG. 1 is a block diagram illustrating a conventional method of manufacturing an ITO sintered body based on the method described above.
Steps 10, 15 and 20 in FIG. 1 show a precipitation by adding alkali to the solution of indium salts and tin salts. Step 25 shows an amount of indium oxide powder and tin oxide powder which is mixed by filtering, drying and calcining the hydroxide precipitated in step 20. Step 30 shows ball milling of the mixed powder of indium oxide and tin oxide, i.e., ITO powder, which was produced in step 25. Step 35 shows press molding or cold isostatic press (CIP) molding of the ball-milled powder. Step 40 shows sintering the press-molded or CIP-molded body. Step 45 shows machine working of the sintered body, by which an ITO sintered body target is produced.
However, the density of an ITO sintered body composed of the indium oxide powder and tin oxide powder, which are processed by the conventional method described above, reaches merely 65% (4.65 g/cm.sup.3) of the theoretical density stated above. In other words, an ITO sintered body processed by the manufacturing method illustrated in FIG. 1 possesses low electrical conductivity (specific resistivity), low thermal conductivity and low fracture strength. The reason is because the indium oxide powder and tin oxide powder, which are major constituents of an ITO sintered body, are processed with low density.
Diverse methods have been developed to manufacture a high density ITO sintered body and ITO target by employing the indium oxide powder and tin oxide powder that are processed by the method described above.
Japanese Patent No. 58-136480 discloses a method of manufacturing a high density ITO sintered body and ITO target. This method, known as "A Hot Press Sintering Method", is a method of sintering the ITO powder comprising indium oxide powder and tin oxide powder under a temperature range of about 900.degree. C. to 1,200.degree. C. in a graphite mold either in a vacuum or filled with inert gas.
U.S. Pat. No. 5,094,787 discloses a method of manufacturing a high density ITO sintered body by press molding the ITO powder under a pressure of about 1,000 kg/cm.sup.2 and sintering the molded body under 1 to 10 oxygen atmospheric pressure.
Another method of manufacturing a high density ITO sintered body employs a hot isostatic press (HIP).
U.S. Pat. No. 4,962,071 discloses a sintering method of ITO powder of high density by adding an additive which is liquified at a sintering temperature (e.g., Al.sub.2 O.sub.3 +SiO.sub.2).
Japanese Patent Laid-Open No. 5-17201 discloses a method of manufacturing an ITO sintered body by partially deoxidizing molded ITO powder at a high temperature or by discharging the powder for a pretreatment.
A method of manufacturing ITO powder or indium oxide powder of high sinterability other than a method of improving the sintering process is disclosed in EP 0386932, U.S. Pat. Nos. 5,417,816 and 5,401,701 in which an ordinary sintering furnace was used to manufacture a high density ITO sintered body under ordinary atmospheric pressure (1 atmospheric pressure).
FIG. 2 is a block diagram illustrating the process of manufacturing an ITO sintered body disclosed in U.S. Pat. No. 5,417,816. In this process, the addition of alkali and the hydroxide precipitation stop illustrated in FIG. 1 (step 15) are replaced with the precipitation of the solution of indium salts and tin salts by means of In, Sn and In--Sn electrodes (step 16). Other steps in FIG. 2 that are identical to those illustrated in FIG. 1 will be excluded from the present description for the sake of brevity.
The manufacturing methods described above have drawbacks in terms of economy, characteristics of the resulting sintered body, productivity of sputtering, and characteristics of the ITO film. In other words, manufacturing an ITO sintered body according to the conventional method results in sintered bodies of different densities, different production costs and different characteristics of ITO targets depending on the manufacturing method that is applied.